In 1987, nitric oxide (.NO), a gaseous free-radical, was discovered in humans (see, for example, Ignarro et al., in Proc. Natl. Acad. Sci., USA 84:9265-69 (1987) and Palmer et al., in Nature 327:524-26 (1987)). As an indication of the significance of this discovery for the understanding of human physiology and pathophysiology, Science magazine selected nitric oxide as the molecule of the year in 1992.
Nitric oxide is formed from the terminal guanidino nitrogen atom of L-arginine by nitric oxide synthase (NOS; see, for example, Rodeberg et al., in Am. J. Surg. 170:292-303 (1995), and Bredt and Snyder in Ann. Rev. Biochem. 63:175-95 (1994)). Two major forms of nitric oxide synthase, constitutive and inducible enzymes, have been identified.
Under physiological conditions, a low output of .NO is produced by the constitutive, calcium-dependent NOS isoform (cNOS) present in numerous cells, including endothelium and neurons. This low level of nitric oxide is involved in a variety of regulatory processes, e.g., blood vessel homeostasis, neuronal communication and immune system function. On the other hand, under pathophysiological conditions, a high output of .NO is produced by the inducible, calcium-independent NOS isoform (iNOS) which is expressed in numerous cell types, including endothelial cells, smooth muscle cells and macrophages. These high levels of nitric oxide have been shown to be the etiology of endotoxin shock. This high output of .NO further contributes to inflammation-related tissue damage, neuronal pathology, N-nitrosamine-induced carcinogenesis and mutations in human cells and bacteria via deamination reaction with DNA. Nitric oxide can therefore be seen to be a mixed blessing, being very desirable when present in small amounts, while potentially being highly detrimental when produced in excessive quantities.
Nitric oxide is a potent vasodilator (see, for example, Palmer in Arch. Surg. 128:396-401 (1993) and Radomski & Moncada in Thromb. Haemos. 70:36-41 (1993). For example, in blood, .NO produced by the endothelium diffuses isotropically in all directions into adjacent tissues. As .NO diffuses into the vascular smooth muscle, it binds to guanylate cyclase enzyme, which catalyzes the production of cGMP, inducing vasodilation (see, for example, Ignarro, L. J., Ann. Rev. Toxicol. 30:535-560 (1990); Moncada, S., Acta Physiol. Scand. 145:201-227 (1992); and Lowenstein and Snyder, Cell 70:705-707 (1992)). The overproduction of nitric oxide causes an extreme drop in blood pressure, resulting in insufficient tissue perfusion and organ failure, syndromes that are associated with many diseases and/or conditions (e.g., septic shock, overexpression of cytokines, allograft rejection, and the like). The overproduction of nitric oxide is triggered by a number of stimuli, such as, the overproduction of inflammatory cytokines (e.g., tumor necrosis factor (TNF), interleukin-1 (IL-1), interferons, endotoxin, and the like). Additionally, the overproduction of .NO has been discovered to be one of the major side-effects of cytokine therapy (see, for example, Miles et al., in Eur. J. Clin. Invest. 24:287-290 (1994) and Hibbs et al., in J. Clin. Invest. 89:867-877 (1992)). Thus, abnormally elevated nitric oxide levels have been linked to many inflammatory and infectious diseases.
Inflammatory cytokines (e.g., TNF, interleukins or interferons) and infectious agents (e.g., endotoxin) induce nitric oxide overproduction by inducing transcription of the inducible nitric oxide synthase gene, leading to the production of inducible nitric oxide synthase, which in turn results in the overproduction of nitric oxide. The production of nitric oxide by the above-described pathway can be disrupted in a variety of ways. Thus, for example, there have been attempts to develop monoclonal antibodies (e.g., anti-endotoxin antibodies, anti-cytokine antibodies, anti-cytokine receptor antibodies, and the like) in efforts to block the .NO production pathway at the transcriptional level. Unfortunately, however, such efforts have met with very limited success (see, for example, Glauser et al., in Clin. Infect. Dis. 18:S205-16 (1994) and St. John & Dorinsky, in Chest 103:932-943 (1993)). At least one reason for the relative lack of success in the art is the fact that the production of inflammatory cytokines is short-lived (see, for example, Wange & Steinsham in Eur. J. Haematol. 50:243-249 (1993)), while overproduction of nitric oxide lasts several days, causing systemic hypotension, insufficient tissue perfusion and organ failure.
Thus, for example, during endotoxemia, TNF production peaks at about 1-2 hours. Therefore, in order to be effective, anti-TNF antibodies would have to be administered at an early stage after infection. Indeed, in many clinical settings, patients are likely to already have been infected with bacteria prior to being admitted. Accordingly, such therapeutic methods have met with only limited success.
Currently, many pharmaceutical companies have turned their attention to the design and development of substrate or product analogue inhibitors of the enzyme, NOS, in efforts to treat the overproduction of .NO. However, recent data show that the inhibition of NOS is detrimental to subjects. For example, rodent studies show that inhibition of the production of nitric oxide causes intrauterine growth retardation and hind-limb disruptions in rats (see, for example, Diket et al., in Am. J. Obstet. Gynecol. 171:1243-1250 (1994)). Furthermore, the inhibition of nitric oxide synthesis during endotoxemia has also been shown to be detrimental (see, for example, C. O. Corso et al., J. Hepatol. 28:61-69, 1998; K. Kaneda et al. Acta Anaesthesiol. Scand. 42:399-405, 1988; R. I. Cohen, et al. Crit. Care Med. 26:738-747, 1998. Similar results have been reported in larger animal studies, such as dogs and swine (see, for example, Statman et al., in J. Surg. Res. 57:93-98 (1994); Mitaka et al., Am. J. Physiol. 268:H2017-H2023(1994); Robertson, et al., Arch. Surg. 129:149-156 (1994); and Henderson et al., Arch. Surg. 129:1271-1275 (1994)).
Dithiocarbamates such as pyrrolidine dithiocarbamate have been determined to be potent inhibitors of nuclear factor kappa B (NF.kappa.B) in intact cells (see, for example, R. Schreck et al., in J. Exp Med 175:1181-1194 (1992). In addition, NF.kappa.B has also been shown to up-regulate the expression of cell adhesive molecules, including the vascular cell adhesive molecule-1 (VCAM-1; see, for, example, M. F. Iademarco et al., J. Biol Chem 267:16323-16329 (1992)). Interestingly, in view of these known inhibitory effects of dithiocarbamates on NF.kappa.B, and the known ability of NF.kappa.B to induce expression of VCAM-1, Medford et al. propose the allegedly new use of dithiocarbamates to treat cardiovascular diseases mediated by VCAM-1, through the inhibition of the NF.kappa.B pathway (see U.S. Pat. No. 5,380,747).
It is also beneficial to remove cyanide (CN), a fast acting toxic compound, from subjects exposed thereto. Cyanide is frequently used in suicides, homicide, and chemical warfare (see, for example, Salkowski et al., in Vet. Hum. Toxicol. 36:455-466 (1994) and Borowitz et al., in B. Somani (Ed.), Chemical Warfare Agents, Academic Press, New York, pp. 209-236 (1992)). Cyanide toxicity can arise from a variety of sources, e.g., from inhalation of smoke produced by the pyrolysis of plastics or nitrile-based polymer fibers, materials that are commonly used in construction and for furniture manufacture. Cyanide toxicity can also occur from ingestion of plant extracts containing cyanogenic glycosides (such as cassava), or from inhalation of airborne vapors encountered in industrial or occupational settings (for example, during electroplating). Clinically, the release of cyanide from sodium nitroprusside (see, for example, Vessy and Cole, in Br. J. Anaesth. 57:148-155 (1985)) and laetrile (see, for example, Sadoff et al., in J. Am. Med. Assoc. 239:1532 (1978)) can create a life-threatening situation.
Acute cyanide poisoning of mammals is characterized by convulsion, uncoordinated movement, decreased motor activity, coma and respiratory arrest, symptoms indicating that the brain is one major target site for cyanide. This type of neurotoxicity is now known to be caused by cyanide-induced depletion of dopamine (see, for example, Kanthasamy et al., in Toxicol. App. Pharmacol. 126:156-163 (1994)) and by an increase in calcium in the brain (see, for example, Yamamoto, in Toxicol. 61:221-228 (1990)). The systemic toxic effect of cyanide has been attributed mainly to its binding to the ferric iron in cytochrome c oxidase, the terminal oxidase enzyme of the mitochondrial respiratory chain. The reaction forms a stable but reversible complex and subsequently disrupts cellular energy production. The reduction of cellular oxygen consumption results in an increase in venous oxygen partial pressure (PO.sub.2).
The classic antidotal action for cyanide poisoning, introduced by Chen et al. in 1933 (see, for example, Chen et al., Proc. Soc. Exp. Biol. Med. 31:250-252 (1933)), involves inhalation of amyl nitrite, followed by intravenous injection of sodium nitrite and sodium thiosulfate. This procedure is still used clinically worldwide, including the United States (see, for example, Dreisbach, in Handbook of poisoning: Diagnosis and treatment, 12th edn., Lange Med. Publications., Los Altos, Calif., p.251 (1987)). In essence, in this method, oxyhemoglobin in red blood cells in the circulation is converted into methemoglobin by chemical reaction with nitrites. Methemoglobin then binds cyanide, thereby removing it from the circulation. Sodium thiosulfate is used as a sulfur donor to allow the formation of thiocyanate, through the reaction catalyzed by rhodanese enzyme (see, for example, Baskin et al., in J. Clin. Pharmacol. 32:368-375 (1992)).
There are, however, major drawbacks of the nitrite/sodium thiosulfate method. For example, the rate of methemoglobin formation is quite slow, taking up to 20 minutes to produce sufficient amounts of methemoglobin. Moreover, the formation of methemoglobin compromises the oxygen-carrying capacity of red blood cells. This is particularly undesirable for victims of smoke inhalation, as adequate ventilation and blood oxygenation are particularly crucial for survival in such situations. Furthermore, hypotension induced by the treatment (i.e., nitrite-induced hypotension) can be life-threatening.
In addition to nitrites, a variety of chemical agents have been used to induce methemoglobinemia as a treatment for cyanide poisoning. These include primaquine phosphate, 6-methoxy-8-(6-diethylamino-hexylamino)lepidine dihydrochloride, p-aminooctoyl-phenone, p-aminopropiophenone, hydroxylamine, 4-dimethylaminophenol, and the like (see, for example, Scharf et al., in Gen. Pharmacol. 23:19-25 (1992)). Although the rates of methemoglobin formation induced by these agents are faster than those produced by nitrites, the same problems as described above are common to all methemoglobin formers.
Recently, hydroxocabalamin, vitamin B.sub.12, has been shown to be effective in the treatment of cyanide poisoning in smoke inhalation (see, for example, Houeto et al., in Lancet 346:605-608 (1995)). Hydroxocabalamin is a cobalt-containing compound for which only minute amounts are needed physiologically. Clinical use of hydroxocabalamin for the treatment of cyanide poisoning, however, requires the use of 5 grams per patient. Such high levels of hydroxocabalamin are not only expensive but also potentially toxic because extremely high circulatory levels of cobalt are produced.
Nitroprusside (SNP for sodium nitroprusside) is widely used as a source of nitric oxide for the treatment of severe hypertension, induction of arterial hypotension during surgery, the reduction of after-load after myocardial infarction and during severe congestive heart failure (see, for example, Rokonen et al., in Crit. Care Med. 21:1304-1311 (1993) and Sellke et al., in Circulation 88:II395-II400 (1993)). A nitroprusside molecule (NaFe(CN).sub.5 NO.2H.sub.2 O) contains one nitric oxide and five cyanide groups. Upon intravenous infusion, nitroprusside is known to be metabolized through one-electron reduction to release nitric oxide, a potent vasodilator, which exerts the desired antihypertensive effect (see, for example, Bates et al., in Biochem. Pharmacol. 42:S157-S165 (1991) and Kowaluk et al., in J. Pharm. Exp. Therap. 262:916-922 (1992)). Unfortunately, however, upon release of nitric oxide, SNP further decomposes to release five cyanide groups which can produce life-threatening cyanide poisoning in patients. This high level of cyanide release occurs very commonly in high dose or prolonged therapy with nitroprusside.
Current clinical treatment of nitroprusside-induced cyanide toxicity is, unfortunately, limited to the use of amyl nitrite and sodium nitrite (for the conversion of hemoglobin to methemoglobin) or vitamin B.sub.2. The many drawbacks of using these agents have been set forth above.
Another chemical species whose effect can be detrimental when levels arise above physiological levels is iron. Iron is crucial for maintaining normal structure and function of virtually all mammalian cells (see, for example, Voest et al., in Ann. Intern. Med. 120:490-499 (1994) and Kontoghiorghes, G. J., in Toxicol. Letters 80:1-18 (1995)). Adult humans contain 3-5 g of iron, mainly in the form of hemoglobin (58%), ferritin/hemosiderin (30%), myoglobin (9%) and other heme or nonheme enzyme proteins (Harrison and Hoare, in Metals in Biochemistry, Chapman and Hall, New York, 1980).
Total iron levels in the body are regulated mainly through absorption from the intestine and the erythropoietic activity of the bone marrow. Upon absorption, iron is transported to various tissues and organs by the serum protein transferrin. Once transported to the target tissue or organ, iron is transported and stored intracellularly in the form of ferritin/hemosiderin. Under normal conditions, transferrin is about 30% saturated with iron in healthy individuals, and an equilibrium is maintained between the sites of iron absorption, storage and utilization. The presence of these homeostatic controls ensures the maintenance of physiological levels of not only iron, but also other essential metal ions such as copper, zinc and cobalt.
Breakdown of these controls could result in metal imbalance and metal overload, causing iron overloading toxicity and possibly death in many groups of patients, especially those with idiopathic hemochromatosis (see, for example, Guyader et al., in Gastroenterol. 97:737-743 (1989)). Among its toxic effects, iron is known to mediate a repertoire of oxygen related free radical reactions (see, for example, Halliwell and Gutteridge, in Halliwell and Gutteridge, Free Radicals in Biology and Medicine, 2nd edition. Oxford: Clarendon Press, 15-19 (1989)). For example, iron, particularly in the form of free iron ions, can promote the generation of reactive oxygen species through the iron-catalyzed Haber-Weiss reaction (see, for example, Haber and Weiss, in Proc. R. Soc. Ser. A. 147:332 (1934)) as follows: EQU Fe.sup.3+ +.O.sub.2.sup.- .fwdarw.Fe.sup.2+ +O.sub.2 EQU Fe.sup.2+ +H.sub.2 O.sub.2 .fwdarw.Fe.sup.3+ +.OH+OH.sup.-
The net result of these reactions is as follows: EQU .O.sub.2.sup.- +H.sub.2 O.sub.2 .fwdarw..OH+OH.sup.- +O.sub.2.
The Haber-Weiss reaction is seen to produce the hydroxyl radical (.OH), a highly potent oxidant which is capable of causing oxidative damage to lipids, proteins and nucleic acids (see, for example, Lai and Piette, in Biochem. Biophys. Res. Commun. 78:51-59 (1977); and Dizdaroglu and Bergtold, in Anal. Biochem., 156:182 (1986)).
The occurrence of iron imbalance resulting in excessive in vivo iron levels can be categorized into two conditions, namely iron-overload and non-iron overload conditions (see, for example, Voest et al., supra; Kontoghiorghes, supra). Iron-overload conditions are common in such patients as those suffering from thalassemia, sickle cell anemia, repeated blood transfusion and hereditary hemochromatosis. In such patients, transferrin is fully saturated with iron, and excess low-molecular-weight iron appears in the serum. This low-molecular-weight iron appears to originate from the iron released mainly from the liver and spleen, and from the breakdown of effete red cells. Other iron overload diseases and conditions include hereditary spherocytosis, hemodialysis, dietary or latrogenic iron intake, intramuscular iron dextran and hemolytic disease of the newborn (see, for example, Voest et al., supra; Kontoghiorghes, supra).
Non-iron overload conditions relate to situations where elevated iron levels are the result of therapeutic intervention, such as, for example, anthracycline anti-cancer therapy or inflammatory diseases such as rheumatoid arthritis. While anthracyclines such as adriamycin (doxorubicin) are effective in the treatment of a number of neoplastic diseases, these compounds have limited clinical utility due to the high incidence of cardiomyopathy (see, for example, Singal et al., in J. Mol. Cell. Cardiol. 19:817-828 (1987)).
The molecular mechanism of cardiomyopathy is now attributed to the adriamycin-induced release of iron from intracellular iron-containing proteins, resulting in the formation of an adriamycin-iron complex, which generates reactive oxygen species causing the scission and condensation of DNA, peroxidation of phospholipid membranes, depletion of cellular reducing equivalents, interference with mitochondrial respiration, and disruption of cell calcium homeostasis (see, for example, Myers et al., Science 197:165-167 (1977); and Gianni et al., in Rev. Biochem. Toxicol. 5:1--82 (1983)). On the other hand, several clinical studies have shown that patients with rheumatoid arthritis exhibit elevated low-molecular weight iron species and ferritin-bound iron levels in synovial fluid. Iron, presumably via its mediation of oxygen free radical pathways, exerts its proinflammatory effects in rheumatoid arthritis (see, for example, Muirden and Senator, in Ann. Rheum. Dis. 27:38-48 (1968); and Biemond et al., in Arthritis Rheum. 29:1187-1193 (1986)).
Iron also plays an important role in many aspects of immune and nonimmune host response (see, for example, De Sousa et al., in Ann. N.Y. Acad Sci. 526:310-323 (1988)). It is known that increased concentrations of iron are deleterious to the immune system through the initiation or maintenance of inflammatory reactions (see, for example, Biemond et al., in J. Clin. Invest. 73:1576-9 (1984), and Rowley et al., in Clin. Sci. 66:691-5 (1984)). Other non-iron overload diseases and conditions include reperfusion injury, solid tumors (e.g., neuroblastoma), hematologic cancers (e.g., acute myeloid leukemia), malaria, renal failure, Alzheimer's disease, Parkinson's disease, inflammation, heart disease, AIDS, liver disease (e.g., chronic hepatitis C), microbial/parasitic infections, myelofibrosis, drug-induced lung injury (e.g., paraquat), graft-versus-host disease and transplant rejection and preservation.
Hence, not surprisingly, there has been a tremendous interest in the therapeutic use of chelators in the treatment of both iron-overload and non-iron overload diseases and conditions. A chelator (Greek, chele-claw of a crab) is a molecule forming a cyclic ring with a metal as the closing member. Hundreds of chelating agents have been designed and developed for animal and human studies. Among them, at least fifteen different chelators have been used in humans, including desferrioxamine (DF), ethylenediamine-tetraacetic acid (EDTA), diethylenetriaminc pentaacetic acid (DTPA), pyridoxalisonicotinoylhydrazone (PIH), 1,2-dimethyl-3-hydroxypyrid-4-one (L1) and [+]1,2-bis-(3,5-dioxopiperazine-1-yl) propane (ICRF-187).
For the past 30 years, DF (i.e., desferrioxamine) has been the most commonly used chelating drug for the treatment of transfusional iron overload (see, for example, Pippard et al., in Blood 60:288-294 (1982); Proper et al., in N. Engl. J. Med. 294:1421-1423 (1976); and St. Louis et al., in Lancet 336:1275-1279 (1990)). Patients suffering from thalassemia lived longer with the DF treatment. However, major drawbacks in the use of DF include the cost thereof (.about.$7,000/ patient/year), which can be affordable only by a very small percentage of thalassemia patients worldwide. Another drawback to the use of DF includes the toxicity thereof, including ophthalmic and auditory toxicities as well as induction of pulmonary and renal damage.
Unlike DF, L1 (i.e., 1,2-dimethyl-3-hydroxypyrid-4-one) and related compounds are orally available iron chelators, showing promise in improving the quality of life in patients with thalassemia (see, for example, Olivieri et al., in Drugs Today 28(Suppl. A):123-132 (1992)) and rheumatoid arthritis (see, for example, Vreugdenhil et al., in Lancet 2:1398-9 (1989)). However, the major side effects of L1 therapy include myclosuppression, fatigue, and maternal, embryo and teratogenic toxicity, which severely limits the potential clinical applications thereof (see, for example, Kontoghiorghes, in Int. J. Hematol. 55:27-38 (1992)).
Recently, ICRF-187 has been demonstrated to be effective in removing iron from the anthracycline-iron complex, therefore preventing the cardiac toxicity in cancer patients receiving adriamycin chemotherapy (see, for example, Kolaric et al., in Oncology 52:251-5 (1995)). However, when chelated with iron, the iron-ICRF-187 complex per se is also very effective in the promotion of hydroxyl radical generation via the Fenton reaction, causing oxidative damage to tissues (see, for example, Thomas et al., in Biochem. Pharmacol. 45:1967-72 (1993)). In addition, since ICRF-187 is a strong chelator (having a structure similar to EDTA), it chelates not only low-molecular-weight iron, but also chelates iron from transferrin and ferritin, as well as copper from ceruloplasmin, thus potentially affecting normal cellular iron metabolism.
Another major complication in the therapeutic use of chelators is the propensity of chelators to affect not only the desired metal but also many other essential metals, their associated metabolic pathways and other processes. Thus, for example, the treatment with DF and L1 requires zinc supplementation to prevent the occurrence of zinc deficiency diseases (see, for example, De Virgilis et al., Arch. Dis. Chil. 63:350-255 (1988); and Al-Refai et al., Blood 80:593-599 (1992)).
The low-molecular-weight iron pool in serum is thought to be the most labile iron source during chelation therapy. Chelators that remove this low molecular weight iron with only a minimal effect on other essential metal contents in the body are highly desirable, particularly for the treatment of transfusion-induced iron overload, as well as iron overload induced by anthracycline anti-cancer agents, inflammatory diseases such as rheumatoid arthritis, multiple sclerosis, and the like.
Chronic exposure of the skin to sunlight or ultraviolet radiation can cause severe damage to the underlying connective tissue, leading to erythema and other skin diseases (see, for example, Beisset and Granstein in Crit. Rev. Biochem. Mol. Biol. 31:381-404 (1995) and Kaminester in Arch. Fam. Med. 5:289 (1996). Although the mechanism by which photodamage occurs is not well understood, reactive oxygen species (such as singlet oxygen, superoxide and hydrogen peroxide) and reactive nitrogen species (such as nitric oxide and peroxynitrite) have been implicated as important contributors to such damage (see, for example, Jurkiewicz and Buettner in Photochem. Photobiol. 59:1-4 (1994), Deliconstantinos et al., in Biochem. Pharmacol. 51:1727-1738 (1996) and Deliconstantinos et al., in Brit. J. Pharmacol. 114:1257-1265 (1995)). The skin is known to contain high levels of iron (see, for example, Bissett et al., in Photochem. Photobiol. 54:215-223 (1991). Upon release intracellularly by ultraviolet radiation, iron can participate in oxygen radical formation, thus enhancing the likelihood of causing photodamage, and enhancing the level of photodamage which actually occurs. For example, the combination topical application of the iron chelator, 2-furildioxime, in combination with sunscreen, has been shown to produce synergistic photoprotection (see, for example, Bissett et al., in J. Am. Acad. Dermatol. 35:546-549 (1991)). However, further development in the field is needed to produce more effective and safer iron chelators for the prevention of photoaging and photodamage.
Since a variety of stimuli induce expression of nitric oxide synthase, which, in turn, leads to nitric oxide overproduction (with its attendant detrimental effects), there is a need in the art to effectively treat both the initial stimulus of nitric oxide synthase expression, and the resulting overproduction of nitric oxide, as well as overproduction of nitric oxide which may be induced (directly or indirectly) by therapeutic agents employed for the treatment of a wide variety of infectious and/or inflammatory conditions. There is also still a need in the art for effective, rapid acting, non-toxic antidotes for cyanide poisoning and for new iron scavengers that are capable of removing free iron ions from body fluids, without affecting the normal cellular iron metabolism.